The present invention relates to an arrangement for recirculation of exhaust gases of a supercharged combustion engine according and to controlled cooling of the recirculating exhaust gases.
The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. Supplying a largest possible amount of air to the combustion engine entails cooling the compressed air in a charge air cooler before it is led to the combustion engine. The compressed air is cooled, usually in a charge air cooler situated at a front portion of a vehicle, by surrounding air. The compressed air can thus be cooled to a temperature substantially corresponding to the temperature of the surroundings.
The technique known as EGR (Exhaust Gas Recirculation) is a known way of recirculating part of the exhaust gases from a combustion engine. The recirculating exhaust gases are mixed with the inlet air to the combustion engine before the mixture is led to the cylinders of the combustion engine. Adding exhaust gases to the air causes a lower combustion temperature resulting inter alia in a reduced content of nitrogen oxides NOx in the exhaust gases. This technique is used for both Otto engines and diesel engines. The recirculating exhaust gases are cooled, usually in an EGR cooler, before they are mixed with the inlet air. Coolant-cooled EGR coolers are commonly used, but the use of air-cooled EGR coolers is also known. In an air-cooled EGR cooler the recirculating exhaust gases are cooled to a temperature substantially corresponding to the temperature of the compressed air. The recirculating exhaust gases therefore do not warm the cooled compressed air when they are mixed and led to the combustion engine.
Exhaust gases contain a relatively large amount of water vapour. When they are cooled to a temperature below its dewpoint, the water vapour will condense within the EGR cooler. In situations where the temperature of the surrounding air is below 0° C., there is also risk of condensed water vapour freezing to ice within the EGR cooler. Such ice formation entails the possibility of the exhaust flow ducts through the EGR cooler becoming more or less obstructed, resulting in cessation of the recirculation of exhaust gases.
U.S. Pat. No. 6,367,256 refers to a combustion engine with a system for recirculation of exhaust gases, in which the recirculating exhaust gases are cooled in a coolant-cooled EGR cooler. The coolant flow through the EGR cooler is constant and large so that local boiling of the coolant in the EGR cooler is prevented even in situations where large amounts of exhaust gases are recirculated. When a smaller amount of exhaust gases is recirculated or the coolant is at a low temperature, the ample coolant flow in the EGR cooler may cool the recirculating exhaust gases to such a low temperature that the water vapour in the exhaust gases condenses. To prevent the returned exhaust gases from reaching too low a temperature, the recirculating exhaust gases are led entirely or partly through a bypass line instead of being cooled in the EGR cooler. The system comprises condensate separation devices at various points along the exhaust flow path. An object of that invention seems to be to prevent or at least reduce condensate precipitation along the exhaust flow path.